From the first inclusion of color into banknotes to the printing of modern color-shifting inks, the use of color has long been an effective tool in the fight against counterfeiting. Photonic crystals, similar to precious opal gemstones, display bright reflections of color due to a three-dimensional ordered internal structure.
Photonic crystals are materials having a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which the propagation of electromagnetic waves within certain ranges of wavelengths is inhibited or restricted. The positions of these bands are dependent on the distance between the periodic modulations in the crystal. The reflected stop band wavelengths can appear in the reflectance spectrum as a distinct reflectance peak known as a Bragg peak. The crystal may have a one-, two-, or three-dimensional (3-D) periodic structure.
A three-dimensional photonic crystal has an ordered periodicity in all three dimensions. Such a structure has stop bands for incident light in all directions. Methods for manufacturing these three-dimensional photonic crystals include holographic writing/curing followed by etching and self-assembly of spherical microparticles. Such photonic crystals may also be inverse crystals, in which the periodic structure of the crystal comprises a periodic array of spherical voids.
An inverse photonic crystal can be manufactured using a colloidal photonic crystal template. The three-dimensional photonic crystals formed by spherical microparticles are used as a template for infiltration. When the particulate template is removed, the result is an inverse photonic crystal having a three-dimensional ordered array of voids. Such a templating strategy is disclosed in U.S. Pat. No. 6,261,469. The photonic crystal disclosed in this reference is in block form, which may not be suitable in many applications.
When the dimensions of the distances between the three-dimensional periodic modulations of particles or voids are commensurate with the wavelength of visible light, the reflected stop band of the photonic crystals lies within the energy range of visible light. By using particles or void diameters ranging from 150 nm to 1000 nm, colors encompassing the ultraviolet, visible, and infrared range of the electromagnetic spectrum can be generated.
Potential applications of three-dimensional photonic crystal films include separation media, elements of optical computers, data storage media, optical filters, and security features.
Due to the sensitivity of a photonic crystal, slight changes in the refractive index or lattice spacing result in detectable shifts of the reflected stop band. This can be employed where the refractive index or the periodic spacing of the photonic crystal is modulated in response to external stimuli or can be controlled by formulating the photonic crystal material composition or by choosing a specific constituting particle diameter for the photonic crystal or the template structure. Examples of such applications are given in U.S. Patent Application Publication No. 2004/0131799, PCT Application No. PCT/CA2007/000236, and U.S. patent application Ser. No. 11/831,679. Deformable photonic crystal are also known, comprising non-close-packed spheres embedded in an hydrogel or elastomer matrix, for example as discussed in U.S. Pat. No. 6,544,800 to Asher, U.S. Pat. Nos. 5,266,238 and 5,368,781 to Haacke et al., by Holtz et al. in Nature 389:829-832, by Foulger et al. in Advanced Materials 13:1898-1901, by Asher et al. in Journal of the Material Chemical Society 116:4997-4998, and by Jethmalani et al. in Chemical Materials 8:2138-2146.
Examples of photonic crystal structures that can respond to external stimuli include colloidal photonic crystals in the form of optical films (Busch et al., Phys. Rev. E, 58:3896, 1998; Xia et al., Adv. Mater., 12:693, 2000). The stop band wavelength ranges of these materials are highly sensitive to changes in the external environment, optical characteristics, or the structure of the photonic crystal.
An example for a photonic crystal device tuned by electrical fields is described in U.S. Patent Application Publication Nos. 2009/0034051 and 2008/0224103 by Arsenault et al., where the device displays a variable structural color throughout the entire visible spectrum by electrically stimulating the contraction and expansion of the lattice structure.
An example for a photonic crystal device tuned by mechanical compression is discussed in PCT Patent Application Publication No. 2008/098339 by Arsenault et. al., where a photonic crystal device is compressed by mechanical force. The resulting structural change, the compression of the lattice parameter, causes a dynamic blue-shift of the stop band. Using an elastic photonic crystal device material formulation, The three-dimensional structure may then revert to its original state and dimensions upon removing the compression force. Such devices are suitable as overt security features, where user interaction can produce an obvious and visible effect indicating the security and safety of the article. Mechanical, thermal, chemical, electrical, magnetic, electromagnetic, or ultrasonic stimuli are suitable to invoke an observable response or photonic band gap shift of the photonic crystal device. An example of a peelable security device is the Wallet-Seal developed and manufactured by Schreiner MediPharm and Schreiner ProSecure and currently used at Bayer Healthcare as security seal for Levitra packaging (Product & Image Security Newsletter, No. 69, June 2009).